This invention relates generally to the field of optical communications and in particular to a method and apparatus for providing optical wavelength conversion employing cross phase modulation (XPM).
All optical wavelength converters which operate at speeds beyond the limits of electronic devices will be essential components in future Wavelength-Division-Multiplexed (WDM) networks. As was shown in a paper entitled xe2x80x9cWavelength Conversion at 10 GBit/s Using a Semiconductor Optical Amplifierxe2x80x9d which appeared in Photon Technol. Lett., 5, (11), pp. 1300-1303, (1993), J. M. Weisenfeld demonstrated all optical wavelength conversion using semiconductor optical amplifier (SOA) devices exploiting cross gain modulation (XGM) as well as cross phase modulation (XPM). As shown therein, in the XGM scheme a strong input signal and a continuous wave (cw) signal are introduced into a nonlinear element. The input signal is used to saturate the gain of the nonlinear element and thereby modulates the cw signal carrying the new wavelength. In the XPM scheme, a strong input signal is used to modulate both the phase and intensity of a second signal. The modulation of this second signal is then exploited in an interferometric configuration for redirecting the signal from one output to an other.
Different interferometric configurations have been proposed. Some are based on Michaelson (MI), others are based on Mach-Zehnder interferometer (MZI) configurations with the nonlinear elements on one or both branches of the interferometer arms. (See, e.g., K. Tajima, xe2x80x9cAll Optical Switch with Switch Off Time Unrestricted by Carrier Lifetime:; Jpn. J. Appl, Phys. Vol., 32, No. 12A, pp. L1746-1749; Dec. 1993; K. E. Stubkjaer, T. Durhuus, B. Mikkelsen, C. Joergensen, R. J. Pedersen, C. Braagaard, M. Vaa, S. L. Danielsen, P. Doussiere, G. Garabedian, C. Graver, A. Jourdan, J. Jacquet, D. Leclerc, M. Erman, and M. Klenk, xe2x80x9cOptical Wavelength Convertersxe2x80x9d; Proc. European Conf. on Opt. Communication, Firence, Italy, Vol., 2, 635-642, Sept. 1994; J. M. Weisenfeld, xe2x80x9cWavelength Conversion for Optical Networksxe2x80x9d, Second Optoelectronic and Communications Conference (OECC""97), Technical Digest, pp. 426-427, July 1997. Recent developments include hybrid wavelength converters, using only a single SOA followed by a delay-interference section, formed by a calcite crystal. (See, Y. Ueno, S. Nakamura, K. Tajima, S. Kitamuraxe2x80x9d, xe2x80x9c3.8 THz Wavelength Conversion of Picosecond Pulses Using a Semiconductor Delayed-Interference Signal-Wavelength Converter (DISC)xe2x80x9d, Photon. Technol. Letters, Vol., 10, No. 3, March 1998; Y. Ueno, K. Tajima, xe2x80x9cWavelength Converterxe2x80x9d, EP 0 875 782 A2.
Despite these advances however, these delay interference wavelength converter schemes is hybrid in nature and their operation is fairly limited.
We have developed an integrated wavelength converter with a monolithically integrated delay loop in a delayed interference configuration that needs only one SOA or other non-linear optical element coupled to the input fiber, a first coupler to arranged to split the output of the SOA or other non-linear optical element (i.e., an element that changes its material property, such as, for example, refractive index, absorption or gain, in the presence of a strong light signal) into two paths having controllable delay and phase shift characteristics, and at least one output coupler to combine the signals present on the two paths to provide the converter output. Unlike prior-art hybrid wavelength converters, one embodiment of our inventive device has a monolithically integrated delay loop utilizing a coupler that has an asymmetric splitting ratio. The asymmetric coupler can be either the first coupler or the second coupler, or both. The non-linear optical element can, in addition to a semiconductor optical amplifier, be, for example, an electro-absorption modulator, a DFB laser, a gain clamped semiconductor optical amplifier, etc.
Another embodiment of our invention has a coupler that does not require an asymmetric splitting ratio, and has either a gain element in one or both paths, an attenuation element in one or both paths, or the two in one or both paths.
If desired, yet another coupler can be added to the wavelength converter to couple out part of the light in one or both of the paths, thereby obtaining a better extinction ratio.